Modèle hydrologique du Lac Tchad

Résumé

Le Lac Tchad est modélisé par un bilan de volume donnant ses niveaux et superficies de plans d'eau en fonction d'apports et pertes: débit des tributaires; pluie; évaporation; infiltration. Il est représenté par trois réservoirs (cuvette nord et archipel connectés à la cuvette sud) dont les niveaux de plan d'eau, distincts en période d'apports déficitaires (situation actuelle), sont confondus en période excédentaire (situation avant 1973). Suite aux observations de terrain, la cuvette nord est représentée avec un “réservoir sol” devant être rempli pour que l'eau puisse apparaître en surface. Le modèle utilise sept paramètres optimisés: intensité d'infiltration; profondeur du “réservoir sol”; quatre paramètres d’échange entre réservoirs; niveau de fond de l'archipel. Grâce aux données mesurées au sol ou par satellite (débits, précipitations, évaporation, niveaux et superficie de plans d'eau) il est calé sur la période 1970–1996 et validé sur la période 1956–2008.

Citation Bader, J.-C., Lemoalle, J. & Leblanc, M. (2011) Modèle hydrologique du Lac Tchad. Hydrol. Sci. J. 56(3), 411–425.     

A multi-subwaveform parametric retracker of the radar satellite altimetric waveform and recovery of gravity anomalies over coastal oceans

The quality of satellite radar altimetric data is very important in studies of geodesy,geophysics,and oceanography.Over coastal oceans,altimeter waveforms are contaminated by the terrain and physical environments so that the accuracy of altimeter data is lower than that over open oceans.Here we develop a new multi-subwaveform parametric retracker(MSPR) to improve the quality of altimeter data for the recovery of gravity anomaly in coastal oceans.The least squares collocation method is used to recover the residual gravity anomaly over the coastal water from altimetric data.The waveform data records from Geosat/GM around Taiwan Island are practically retracked with MSPR.When compared with the Taiwan geoid height,the results retracked by MSPR are more accurate than those retracked by the well-known β-5-parmeter method and from the geophysical data records(GDRs).The gravity anomalies over Taiwan coastal waters are calculated from the retracked altimeter data with the least squares collocation.When we compared gravity anomalies computed using altimeter GDRs with the ship-borne gravity data over Taiwan coastal ocean,we found that the results from retracked data are more accurate than those from GDRs.     

ANALYSIS ON STRUCTURAL FEATURES:INSIGHTS FROM SATELLITE-DERIVED GRAVITY MODEL AND SEISMIC PATTERN

In this paper, based on a large number of cumulative observational data from the seismic monitoring network in China, we grid the research area to calculate the density values at each grid node and convert the qualitative earthquake epicenter distribution to quantitative seismic pattern. Minimum magnitude of completeness(M_C)is determined by magnitude-rank analysis, which provides lower limit earthquake and original time. New satellite-derived gravity model v23.1, which is based on satellites CryoSat-2 and Jason-1 data, is used to determine the Bouguer gravity anomaly derived from free-air gravity anomaly and elevation database sets SRTM30, and ultimately, the complete Bouguer correction is obtained. In this paper, the Xingtai earthquake zone and Tanlu fault zone (Anhui segment) are selected for case study. Bouguer gravity anomaly presents a NE-trending U-shaped narrow strip in the Xingtai earthquake zone, and its location is consistent with Shulu Fault Basin. Grid density value contours are restricted by the U-shaped strip, and the extreme value of seismic activity density lies in the bottom of the U-shaped strip as shown in the cross section. The results of Bouguer gravity anomaly and upward continuations to the different heights show good linearity and gradient in the Tanlu fault zone (Anhui segment); and both long-axis direction of seismic pattern and nodal plane strike of seismogenic fault from focal mechanism solutions trend NNE. In short, the Tanlu fault zone(Anhui segment)is a large deep-seated fault that still has the ability to control seismic activity along it. Based on the measured gravity and magmatic data, using the edge detection TDX method to interpret the concealed boundary of the Anqing M4.8 earthquake near the Tanlu fault, and combining with the results from deep seismic reflection profiles of the study area, we discussed the causative fault of the Anqing earthquake.     

����GPS���ջ������źŲ���ˮ��߶�

???????????????????GPS????????????????????????????????GPS???????????????????????????????????????????÷??????????????????????????????????????????????????????????????????????????????????????Lomb??Scargle???????????????????????????????????????????????ù???????????÷????????????????????????3 cm?????????????????????????????鳱??????μ?????     

����ȫ��������ģ�ͺͺ����ģ��ȷ��ƽ������λ����

???ò????????????EGM2008??EGM96???????????????CLS01??DNSC08??WHU2009??CNES_CLS10?????????????????????????μ???????????λ???????????????θ????????λ????????62 636 858.333 3 m ² s ^-2 ?? 62 636 858.260 6 m ² s ^-2??????????EGM2008??????????????λ???С??1.40 m ² s ^-2?????????????????????????????????     

联合卫星重力和卫星测高确定南极绕极流

联合基于GRACE重力卫星观测资料恢复的重力场模型(EIGEN-GL04S1)和卫星测高推求的平均海面高模型(KMSS04)来构造南极绕极流区域的平均海面动力地形,并利用小波滤波方法去掉短波及噪声信号,进而推算大、中尺度的绕极流。与非卫星重力场模型、同化资料及海洋水文资料确定相应结果的验证分析表明:基于新的卫星重力场模型推算的南极绕极流区域的海面动力地形、PF、SAF和表层流场等都与海洋学结果相吻合,且局部特征更加清晰。表明卫-卫跟踪重力卫星计划确定的地球重力场模型较之以前存在的重力场模型在中长波部分精度有较大提高,从大地测量(从空间)角度来探测南极绕极流已达到较高的精度。     

Steric and mass-induced Mediterranean sea level trends from 14 years of altimetry data

Long-term series of almost 14 years of altimetry data (1992–2005) have been analysed along with Sea Surface Temperature (SST) and temperature and salinity profiles to investigate sea level trends over the Mediterranean Sea. Although sea level variations are mainly driven by the steric contribution, the mass-induced component plays some role in modulating its oscillation. A spatially averaged positive trend of 2.1 ± 0.6 mm/year has been observed, but a change in sign in 2001 seems to appear. Steric effects (mainly on thermal origin) account for  55% of sea level trend. Although Mediterranean Sea is a semi-enclosed basin, this value is comparable to that reported for the global ocean. Sea level rise is particularly important in the Levantine basin south of Crete with values up to 10 ± 1 mm/year. Other areas of sea level rise are localised throughout the Levantine basin and in the Adriatic and Alboran Seas, with more moderate values. Sea level drop areas are localised in the Algerian basin, between the Balearic Islands and the African coasts and, particularly, in the Ionian basin. In this area, negative trends as high as − 10 ± 0.8 mm/year are detected mainly due to the mass-induced contribution, which suggests decadal changes of surface circulation. The inferred sea level trends have been correlated with North Atlantic Oscillation (NAO) indices and a low but significant correlation has been detected between sea level in the Levantine and Balearic basins and NAO index.     

Assessment of Radar Waveform Retracked Jason-2 Altimetry Sea Surface Heights Near Taiwan Coastal Ocean

This study focuses on assessing the accuracy of 20-Hz waveform retracked Jason-2 (J-2) altimetry sea surface heights (SSHs) in the vicinity of Taiwan by comparisons with the TOPEX/Poseidon (T/P) 10-Hz SSHs and sea level data from the Anping tide gauge. The study areas exhibit high, medium, and low amplitudes of ocean tides and contain diverse bathymetries with depths of 0–4000 m. The performance of Offset Center of Gravity (OCOG), threshold, modified threshold, and ice retrackers was examined by comparing the retracked SSHs with Earth Gravitational Model 2008 (EGM08) geoid via the use of the improvement percentages (IMPs). The results indicate that both altimetry measurements are significantly improved by waveform retracking techniques, with a maximum IMP of 46.6% for T/P and 82.0% for J-2, and the optimal achievement of retrackers is influenced by the characteristics of the study areas. In addition, valid retracked J-2 SSHs are much closer to shorelines than T/P. A comparison of retracked J-2 data with Anping tide gauge records reveals that applying the optimal retracking algorithms reduces the root mean squares of differences and increases the number of valid measurements.     

基于带谐重力场的测高卫星轨道设计

推导了一组测高卫星轨道设计的理论公式,介绍了相关方法,并编写了一套软件。分别以激光测高卫星ICESat和雷达测高卫星HY-2A为例,基于31阶地球带谐重力场,给出了其平均轨道参数与密切轨道参数。轨道模拟显示,卫星经过各自的回归周期之后,其星下点均能精确回到初始位置,并且长半轴、轨道倾角、偏心率和近地点幅角均没有发生长期和长周期的变化,符合参考轨道要求。     

利用测高重力异常建立南中国海海底地形模型

论述了频率域内海深反演的基本原理,简要分析海底地形和海洋重力异常的函数关系。基于未补偿板块响应函数模型,利用测高重力异常和ETOPO1海深模型对南中国海海底地形进行反演,经ETOPO1模型和实测水深检核,反演出的海深模型相对精度在10%左右。该技术不需要船测数据作为控制,需要先验的海深模型作为参考,在浅海以及多海山地区精度较差,并存在过低估计海山峰值的问题。